Method for producing isocyanates

ABSTRACT

The invention relates to a method for preparing isocyanates by phosgenating the corresponding amines, wherein low-boiling secondary components, excess phosgene, and the co-product hydrogen chloride are separated from the crude liquid isocyanate stream, which is obtained after the phosgenation has occurred, within a maximum of 60 minutes, and wherein the crude liquid isocyanate stream is not exposed to temperatures above 250° C. until said separation.

The invention relates to a method for preparing isocyanates byphosgenating the corresponding amines, wherein low-boiling secondarycomponents, excess phosgene, and the co-product hydrogen chloride areseparated from the crude liquid isocyanate stream, which is obtainedafter the phosgenation has occurred, within a maximum of 60 minutes, andwherein the crude liquid isocyanate stream is not exposed totemperatures above 250° C. until said separation.

Isocyanates are manufactured in large amounts and mainly serve asstarting materials for preparing polyurethanes. They are mostly preparedby reacting the corresponding amines with phosgene, in which, inaddition to the isocyanate, hydrogen chloride is also formed.

A common way of manufacturing isocyanates is the reaction of thecorresponding amines with phosgene in the liquid phase. This methodprocedure, also referred to as liquid phase phosgenation (LPP), ischaracterized in that the reaction is typically conducted in an inertsolvent, and that the reaction conditions are selected such that,besides the solvent, at least the amine, isocyanate and phosgenereaction components are at least partially, preferably predominantly,present in the liquid phase under the selected conditions. Some of thehydrogen chloride produced in the reaction as co-product is presentdissolved in the liquid phase and some leaves the reactor in gaseousform. The liquid phase phosgenation can be carried out at varioustemperature and pressure levels. It is possible, for example, to carryout the liquid phase phosgenation at temperatures of 0° C. to 240° C.and pressures of 1 bar to 70 bar; in some cases temperatures up to 300°C. and pressures up to 300 bar are described.

In the liquid phase method, an efficient mixing of amine and phosgene isvery important. For this purpose, static (preferably nozzles) anddynamic (comprising mechanically moving parts) mixing devices are usedin the prior art. Mixer reactors are known from EP 0 291 819 A and EP 0291 820 A and EP 0 830 894 A, which consist of a substantiallyrotationally symmetrical housing, in which the housing has asubstantially rotationally symmetrical mixing chamber with separateinlets for the at least two feed streams and one outlet. The inlet forat least one first substance stream is provided in the axis of themixing chamber and the inlet for at least one second substance stream isconfigured in the form of a multiplicity of nozzles arranged inrotational symmetry with respect to the mixing chamber axis.

To improve mixing, the mixer reactors also comprise at least one rotordisk-stator disk unit and also an impeller to improve the conveyingaction in the mixer reactor in favor of a narrow residence timespectrum. It is also possible to dispense with the impeller in the mixerreactors described above. The pressure in the mixing chamber is therebyincreased in comparison to the technical teaching of EP 0 291 819 A, EP0 291 820 A and EP 0 830 894 A. To convey the precursor and/or productstream, the initial pressure of the reactant stream is exclusively used.A pump effect is no longer applied. It is also possible to install amodified inducer in such a way that the conveying action acts againstthe main conveying direction of the reactant streams or precursorstream. This also causes the pressure in the mixing chamber to increase.The modified inducer is preferably arranged on the same shaft as therotor disks. Both measures therefore cause an increase in pressure inthe mixing chamber in comparison to the technical teaching of EP 0 291819 A, EP 0 291 820 A and EP 0 830 894 A.

Common to all method variants for liquid phase phosgenation is that theexcess phosgene and the hydrogen chloride formed are removed from thecrude isocyanate dissolved in the solvent on completion of the reaction.Said removal is generally carried out such that a gaseous stream largelycomprising the excess phosgene and hydrogen chloride and a liquidstream, comprising, inter alia, the solvent and the desired isocyanateleaves the reactor in which the reaction takes place. Furthermore,common to all method variants of liquid phase phosgenation is that thegaseous stream is separated into hydrogen chloride and excess phosgeneand the latter is generally fed back again to the reaction. The liquidstream comprising the desired isocyanate and the solvent is generallypurified by distillation. The liquid stream leaving the reactor stillcomprises dissolved phosgene and hydrogen chloride according to theprevailing pressure and temperature conditions. These are removed in thedistillation, together with reaction secondary components whoseformation in the reactor cannot be completely avoided. The desiredisocyanate is obtained in high purity by the distillation.

An overview of different variants of the reaction procedure and theworkup and product recovery is given in the application documentsDE-A-102 60 027, DE-A-102 60 093, DE-A-103 10 888, DE-A-10 2006 022 448,US-A 2007/0299279 and the sources cited therein.

Another possibility for preparing isocyanates is the reaction of thecorresponding amines with phosgene in the gas phase. This methodprocedure, commonly referred to as gas phase phosgenation (GPP), ischaracterized in that the reaction conditions are selected such that atleast the amine, isocyanate and phosgene reaction components, butpreferably all reactants, products and reaction intermediates, aregaseous under the selected conditions.

Various methods for preparing di- and/or polyisocyanates by reacting di-and/or polyamines with phosgene in the gas phase are known from theprior art.

GB-A-1 165 831 describes a method for preparing isocyanates in the gasphase in which the reaction of the amine in the vapor phase with thephosgene is carried out at temperatures between 150° C. and 300° C. in atubular reactor equipped with a mechanical stirrer andtemperature-controllable via a heating jacket. The reactor disclosed inGB-A-1 165 831 resembles a thin film evaporator whose stirrer mixes thegases entering the reaction chamber and gases present in the reactionchamber and at the same time covers the surrounding walls of the tubularreactor with the heating jacket in order to prevent a buildup ofpolymeric material on the tube wall, since such a buildup would hamperthe heat transfer. The document does not disclose how the crudeisocyanate obtained by the reactor disclosed is purified to the pureisocyanate and how yield losses may be reduced.

EP-A-0 289 840 describes the preparation of diisocyanates by gas phasephosgenation, wherein this document discloses the reaction of the amineswith the phosgene in a cylindrical chamber without moving parts in aturbulent flow at temperatures between 200° C. and 600° C. and reactiontimes of an order of magnitude of 10-4 seconds. According to theteaching of EP-A-0 289 840, the gas streams are introduced into thereactor at one end of the tubular reactor through a nozzle and anannular gap between the nozzle and mixing tube and thereby mixed.According to the teaching of EP-A-0 289 840, it is essential for theviability of the method disclosed therein, that the dimensions of thetubular reactor and the flow rates in the reaction chamber are fixedsuch that a turbulent flow prevails in the reaction chamber which ischaracterized by a Reynolds number of at least 2500, preferably at least4700. According to the teaching of EP-A-0 289 840, this turbulence isgenerally ensured when the gaseous reaction partner flows through thereaction chamber at a flow rate of more than 90 m/s. The gas mixtureleaving the reaction chamber is passed through an inert solvent, whichis maintained at a temperature above the decomposition temperature ofthe carbamoyl chloride corresponding to the diamine, wherein thediisocyanate dissolves in the inert solvent. The document describes thatthe crude diisocyanate thus obtained can be worked up by distillation,without providing handling instructions for this. The document gives noguideline as to how yield losses and formation of by-products in thecrude isocyanate stream can be avoided.

EP-B-0 593 334 discloses a method for preparing aromatic diisocyanatesin the gas phase in which a tubular reactor is used. Mixing of thereactants is achieved by a narrowing of the walls. The reaction takesplace in a temperature range of 250° C. to 500° C. At the reactoroutlet, the reaction product is converted to the liquid phase bysupplying solvent. The document describes that the crude isocyanate thusobtained can be worked up by distillation, but handling instructions forthis are not given. The document does not disclose any methods in whichyield losses can be avoided or reduced during workup of the crudeisocyanate.

EP-A-0 570 799 relates to a method for preparing aromatic diisocyanatescharacterized in that the reaction of the related diamine with thephosgene is carried out in a tubular reactor above the boiling point ofthe diamine within a mean contact time of the reactants of 0.5 to 5seconds. As described in the document, reaction times that are too longor too short both lead to undesirable solids formation. Therefore, amethod is disclosed in which the average deviation from the mean contacttime is less than 6%. It is also disclosed in this method that a liquidphase comprising, inter alia, crude isocyanate and a gaseous phasecomprising phosgene and hydrogen chloride is obtained by using a solventat the outlet of the reactor. The document does not provide handlinginstructions for purification of the crude isocyanate.

Compliance with the disclosed contact time distribution is achievedfirstly by the fact that the reaction is carried out in a flow tubewhich is characterized either by a Reynolds number of over 4000 or aBodenstein number above 100. According to the teaching of EP-A-0 570799, a plug flow up to approximately 90% is thereby achieved;furthermore, all parts by volume of the stream have substantially thesame flow times such that the lowest possible deviation in the contacttime distribution between the reaction partners occurs due to theapproximately uniform residence times of all parts by volume.

However, according to the teaching of EP-A-0 570 799, the deviation inthe mean contact time in the practical performance of the method is alsolargely determined by the time required for mixing the reactionpartners. EP-A-0 570 799 states that, as long as the reaction partnersare not homogeneously mixed, volumes of gas are still present in thereaction chamber which were not able to come into contact with thereaction partner and therefore different contact times of the reactionpartners are obtained depending on the mixing of the parts by volume atuniform flow times. According to the teaching of EP-A-0 570 799, themixing of the reaction partners should occur within a period of 0.1 s to0.3 s up to a degree of segregation of 10⁻³, where the degree ofsegregation serves as a measure of the incompleteness of the mixing (seee.g. Chem. Ing. Techn. 44 (1972), p. 1051 ff; Appl. Sci. Res. (theHague) A3 (1953), p. 279). EP-A-0 570 799 discloses that, to obtainappropriately short mixing times, known methods based on mixing unitshaving moving or static mixing devices, preferably static mixingdevices, may in principle be used, while according to the teaching ofEP-A-0 570 799 in particular, use of the jet mixer principle affordssufficiently short mixing times.

A liquid stream comprising, inter alia, the crude diisocyanate, inaddition to the solvent used to liquefy the product gas stream andoptionally to stop the reaction, and also a gaseous stream largelycomprising excess phosgene and hydrogen chloride is therefore alsoobtained in the gas phase phosgenation on completion of the reaction.The gaseous stream is generally separated into hydrogen chloride andphosgene and at least part of the phosgene is reused in the reaction.The liquid stream comprising the desired isocyanate, inter alia, leavingthe reactor, is generally purified by distillation to obtain pureisocyanate.

The essential difference in the methods between the liquid phasephosgenation and the gas phase phosgenation is, therefore, the reactionconditions in the reactor. Common to both variants of the method forpreparing isocyanates is that, on completion of the reaction, firstly acrude product comprising the desired isocyanate, inert solvent,secondary components, hydrogen chloride and unreacted phosgene isobtained, which is split into a gaseous and a liquid product stream,where the liquid product stream comprises, inter alia, the solvent andthe crude isocyanate and this liquid stream is generally worked-up bydistillation.

Despite significant improvements in the reactions both in the liquidphase and the gas phase phosgenation, both by improved mixing techniquesand improved reaction procedures, the formation of high boiling and lowboiling reaction secondary components in the reaction is not completelyavoided. In the context of the present invention, all substances orazeotropically boiling substance mixtures whose boiling points are belowthat of the desired isocyanate in the context of the prevailingconditions of pressure and temperature, are referred to as “lowboilers”. All substances or azeotropically boiling substance mixtureswhose boiling points are above that of the desired isocyanate in thecontext of the prevailing conditions of pressure and temperature, arereferred to as “high boilers”. In the context of the present invention,therefore, “low boilers” applies to the co-product hydrogen chloride andunreacted phosgene. In the context of the present invention, reactionsecondary components (i.e. the products of undesired side reactions) arereferred to as “low boilers” if they fulfill the definition of “lowboilers” mentioned above, and are referred to as “high boilers” if theyfulfill the definition of“high boilers” mentioned above. Thesolvent—either already used in the reaction or not added untillater—usually falls into the group of low-boiling substances (but it isnot considered as a “low boiler” in the above sense since it is not areaction secondary component). However, it is also possible to use asolvent which is assigned to the group of high-boiling substances (butit is not considered as a “high boiler” in the above sense since it isnot a reaction secondary component).

In addition to the side reactions occurring in the reactor in bothmethods, side reactions are also observed which may take place in theliquid product stream comprising the crude isocyanate leaving thereactor. This stream leaving the reactor also respectively compriseshydrogen chloride and excess phosgene in addition to the crudeisocyanate, solvent, low boilers and high boilers. In particular,phosgene is a highly reactive molecule which may react further withother components of the liquid product stream until the separation.

A fundamental problem is that relatively long residence times occur inthe course of the workup to obtain the pure isocyanate, which encourageshigh boiler formation from the isocyanate material of value.Furthermore, high boiling secondary components already produced in thereaction pass into the workup, whereby a further high boiler formationfrom the isocyanate material of value is encouraged. These secondarycomponents have the property of reacting with the isocyanates andtherefore to reduce the proportion of isocyanates in anisocyanate/secondary component mixture as is obtained in a phosgenationreactor. This leads to the formation of residues which are enriched inhigh boiling secondary components. The difficult handling qualities andthe typical composition of such a residue are cited, for example, inDE-A-102 60 093.

In WO 2004/056759 A1, for example, the two-stage separation ofisocyanates from an isocyanate/high boiler mixture (stream 1) isdescribed. Stream 2 (bottom) and 3 (distillate) are distributed in aweight ratio of 20:1 to 1:1. In other words, at most 50% of stream 1 aredrawn from the bottom. The solution is concentrated, pumped into akneader dryer and further evaporated therein.

According to WO 2009/027418 A1, the yield losses mentioned can bereduced if the high boiling compounds already present in the crudeisocyanate mixture, such as urea and its conversion products produced byphosgenation or species formed by secondary reactions of isocyanate,e.g. carbodiimide, isocyanurate, uretdione, are separated before orduring the actual distillation sequence for removal of the solvent andthe low boilers using a suitable apparatus concept. It has also beenfound that some of the high boiling components comprising the isocyanatecan be dissociated back into the isocyanate by suitable pre-evaporation,whereby yield losses via the bottom effluent of the distillation columncan be reduced. Furthermore, it is thereby avoided that monomericisocyanate attaches to the high boiling compounds during the workup andthe yield is thereby reduced. The remaining bottom products comprisecarbodiimides present mainly in polymeric form and polynuclearchlorinated secondary components as major constituents.

From the compilation of various patent applications above, it is evidentthat the process variants described therein relate either to suitableapparatus or process operations in the phosgenation reaction and theprovision/supply of reactants or concern variants of the processoperations in the workup of the reaction mixture including theseparation of low boilers, solvent and/or high boilers. All measuresdescribed should serve to enable a stable process operation or to avoidor reduce the formation of yield-reducing secondary components

Despite the many efforts already made to optimize the reaction of amineswith phosgene, there still exists a need to improve this phosgenationreaction with respect to reducing secondary components. In this context,little attention has been paid to date to the reaction section betweenthe reaction and the removal of low-boiling substances and solvent. Ithas now been found, surprisingly, that this method section has apronounced influence on the process operation, particularly from thepoint of view of safety, product quality and cost-effectiveness of themethod.

The improvement of the method according to the invention forphosgenating amines requires an understanding of the correlationsbetween some of the side reactions occurring which are thereforeexplained in more detail below.

For example, two reaction pathways are important for the formation ofcarbodiimides. Firstly, the thermal decomposition of isocyanates may bementioned, in which two isocyanate groups react with each other withelimination of CO2. This reaction can occur at all stages of the processin which isocyanate is present at elevated temperatures.

Secondly, according to H. J. Twitchett (Chemical Society Reviews (1974),3(2), 209-230), the phosgenation of ureas formed as secondary componentsin the phosgenation lead not only to the corresponding isocyanates butalso to carbodiimides; in addition, the formation of variouschlorine-containing structural elements is also possible, e.g.chloroformamidines, chloroformamidine-N-carbonylchloride and/orisocyanide dichlorides, which arise along the reaction pathway asintermediates. Since, by improving the reaction procedure both in theliquid and in the gas phase phosgenation, the formation of ureas isgenerally only of minor importance, this formation of the carbodiimidepathway plays only a minor role. Nevertheless, the formation of urea andthe resulting carbodiimide formation in the presence of phosgene cannotbe completely excluded.

Chloroformamidine may also be formed from carbodiimides by (reversible)addition of hydrogen chloride, which, for example, is present in thereaction mixture of the phosgenation (see e.g.: A. A. R. Sayigh, J. N.Tilley, H. Ulrich, Journal of Organic Chemistry (1964), 29(11),3344-3347) and may in turn further react thermally, for example, to givetrisubstituted guanidine hydrochlorides. Similarly,chloroformamidine-N-carbonyl chlorides are formed from carbodiimides bythe (reversible) addition of phosgene, which, for example, is present inthe reaction mixture of the phosgenation (see e.g.: H. J. Twitchett,Chemical Society Reviews (1974), 3(2), 209-230) and can be thermallycleaved again into carbodiimides and phosgene.

The reactions presented, namely thermal carbodiimide formation andcarbodiimide formation by phosgenation of urea and also the subsequentreaction of carbodiimides with hydrogen chloride and/or phosgene to giveby-products may take place both in the reactor and also in the liquidproduct stream comprising the crude isocyanate leaving the reactor,since in this stream certain amounts of hydrogen chloride and phosgeneare always dissolved according to the prevailing pressure andtemperature conditions at the outlet of the reactor. The subsequentreactions with hydrogen chloride and phosgene are also possible in theworkup and distillation stages if they are not largely removed from theliquid product stream comprising the crude isocyanate.

Despite large excesses of phosgene, good mixing technique and themeasures already discussed from the prior art, the formation ofby-products with urea structures in the reactor cannot be completelyruled out. Since a possible subsequent reaction of ureas leads tocarbodiimide formation, the liquid product stream comprising the crudeisocyanate leaving the reactor also always comprises carbodiimidefractions. The reaction of these carbodiimides with excess phosgenetherefore always leads to certain proportions of secondary componentshaving chloroformamidine-N-carbonyl chloride structural elements. Thisreaction inevitably proceeds, particularly in the liquid phase in thepresence of dissolved phosgene, as long as phosgene is still presentdissolved in the liquid phase.

It is not only the early removal of high boilers that is helpful tominimize yield losses as described in the literature according to theprior art. Also low-boiling substances present in the liquid productstream comprising the crude isocyanate, such as low boilers andparticularly the co-product hydrogen chloride and unreacted phosgene,can lead to yield reductions.

The formation of secondary components, including the carbodiimidesmentioned above and the chlorine-containing species, causes yield lossesand thus economic disadvantages. If chlorine-containing species get intothe process product, this leads moreover to undesirable elevatedchlorine levels in the product. In addition, by-products having phosgenecarrier chloroformamidine-N-carbonyl chloride structural elements whichmay get into virtually phosgene-free phases of the method and eliminatephosgene again under thermal stress, whereby phosgene can get intovirtually phosgene-free phases of the method.

Mindful of the problems mentioned above, the present invention isconcerned with providing a method for preparing isocyanates from thecorresponding amines which is characterized by a low tendency to formsecondary components, which may lead to losses in yield and/or qualityissues and/or phosgene development in virtually phosgene-free phases ofthe method.

The invention therefore relates to a continuous method for preparing anisocyanate by

(i) reacting the corresponding primary amine with phosgene instoichiometric excess in a reaction chamber, wherein the reaction iscarried out either

in the liquid phase in the presence of an inert solvent or

in the gas phase, wherein a stream comprising a liquid inert solvent isadded to the process product after leaving the reaction chamber,

such that a crude product 1 is obtained comprising the desiredisocyanate, inert solvent, secondary components having a boiling pointbelow that of the isocyanate (low boilers), secondary components havinga boiling point above that of the isocyanate (high boilers), hydrogenchloride and unreacted phosgene,(ii) separating the crude product 1 into a liquid product stream 2containing the desired isocyanate and into a gaseous product stream 3,(iii) working-up the liquid product stream 2, wherein inert solvent, lowboilers, high boilers, hydrogen chloride and phosgene are removed fromthe desired isocyanate,wherein the low boilers, the hydrogen chloride and the phosgene areremoved in step (iii) within a period of 30 seconds to 60 minutes,preferably 60 seconds to 50 minutes, particularly preferably 2 minutesto 40 minutes, following the separation of the crude product 1 in step(ii) into the product streams 2 and 3, and the temperature of theproduct stream 2 is always maintained below or equal to 250° C.,preferably between 100° C. and 250° C., particularly preferably between120° C. and 230° C.

Preferred primary amines are those selected from the group consisting ofaliphatic amines (preferably 1,6-diaminohexane, methylamine, ethylamine,propylamine, butylamine, pentylamine and hexylamine, particularlypreferably 1,6-diaminohexane), cycloaliphatic amines (preferablycyclohexylamine, isophorone diamine, 4,4′-diaminodicyclohexyl methane,2,4′-diaminodicyclohexylmethane, 2,2′-diaminodicyclohexylmethane andmixtures of diaminodicyclohexylmethane isomers, particularly preferablyisophorone diamine, 4,4′-diaminodicyclohexylmethane,2,4′-diaminodicyclohexylmethane, 2,2′-diaminodicyclohexylmethane andmixtures of diaminodicyclohexylmethane isomers), araliphatic amines(preferably benzylamine), and aromatic amines (preferably aniline,chloroaniline, toluylenediamine, 1,5-diaminonaphthalene,4,4′-diaminodiphenylmethane, 2,4′-diaminodiphenylmethane,2,2′-diaminodiphenylmethane, mixtures of diaminodiphenylmethane isomersand mixtures of diaminodiphenylmethane isomers and higher homologuesthereof [also generally referred to as e.g. MDA, PMDA, polymer-MDA ordi- and polyamines of the diphenylmethane series, i.e. mixtures of di-and polyamines, which are obtained from the acid-catalyzed condensationof aniline and formaldehyde], particularly preferably toluylenediamine).

The primary amine used in the method according to the invention isespecially preferably toluylenediamine (TDA). TDA is generally obtainedby nitration of toluene to give dinitrotoluene (DNT) and subsequenthydrogenation thereof. An isomeric mixture is preferably used which islargely composed of meta-TDA isomers (m-TDA; i.e. the two amino groupsare in the meta position to each other) and comprises from 78% by weightto 82% by weight 2,4-TDA and from 18% by weight to 22% by weight2,6-TDA, and which may comprise less than 1% by weight of the para-TDAisomer (2,5-TDA). In the method according to the invention for reactingaromatic diamines with phosgene, however, the use of isomeric mixturesof m-TDA with isomeric ratios deviating therefrom and also the separateuse of technically pure 2,4- or 2,6-TDA isomers is also possible. TheTDA may potentially also comprise low amounts of impurities.

Reaction in stoichiometric excess is understood to mean that more thanthe calculated amount of phosgene is used based on the underlyingstoichiometry of the reaction equilibrium. The molar excess of phosgene,based on the primary amino groups present, is preferably between 1.0%and 1000%, particularly preferably between 10% and 500% and especiallypreferably between 50% and 350% of theory.

Reaction chamber is here understood to mean the space in which theconditions for a reaction of primary amine (or intermediates such ase.g. ureas) with phosgene to the desired isocyanate or to a mixture ofthe desired isocyanate and the corresponding carbamoyl chloride areprovided (in the presence of hydrogen chloride, isocyanate and carbamoylchloride are basically in equilibrium with each other). The reactionchamber therefore begins at the point at which amine and phosgene arebrought into contact with each other for the first time under conditionswhich enable a reaction.

The reaction chamber stops in the case of the liquid phase reaction atthe point at which the reaction is terminated by introducing suitablemeasures (e.g. lowering the temperature). The reaction chamber istypically defined by the spatial dimensions of the interior of theapparatus used.

The reaction chamber stops in the case of the gas phase reaction at thepoint at which either the gas stream composed of products, secondarycomponents, any unreacted reactants and intermediates and optionallyadded inert substances is fed into a device for liquefying theisocyanate formed or the reaction is terminated by introducing othersuitable measures (e.g. lowering the temperature).

The reaction chamber is located in both method procedures in a technicaldevice for carrying out chemical reactions, i.e. the reactor. Aplurality of reactors, connected in parallel or in series, may also beused. In the simplest case, the reaction chamber is identical with theinterior volume of the reactor or reactors. It is also conceivable thata reactor comprises a plurality of reaction chambers.

Reaction in the liquid phase is understood to mean that the amines reactin the liquid phase to the isocyanates and in the course of the reactionall of the components present (reactants, products, intermediates,possibly secondary components, possibly inert substances), while passingthrough the reaction chamber, remain in the liquid phase to at least40.0% by weight, preferably at least 55.0% by weight, particularlypreferably 65.0% by weight and especially preferably at least 80.0% byweight, based in each case on the total weight of all the componentspresent in the reaction chamber. A gas phase, if present, is largelycomposed of HCl and phosgene.

Reaction in the gas phase is understood to mean that the amines react inthe gaseous state to the isocyanates and in the course of the reactionall of the components present (reactants, products, intermediates,possibly secondary components, possibly inert substances), while passingthrough the reaction chamber, remain in the gas phase to at least 95.0%by weight, preferably at least 98.0% by weight, particularly preferablyat least 99.0% by weight and especially preferably at least 99.9% byweight, based in each case on the total weight of all the componentspresent in the reaction chamber.

Inert solvents—applicable to the gas and liquid phase reaction—are thosewhich do not react to a significant degree, preferably not at all, withthe components present (reactants, products, intermediates, secondarycomponents and/or co-products) under the prevailing conditions oftemperature and pressure (see below for details). The inert solvent ispreferably selected from at least one solvent from the group consistingof chlorinated aromatic hydrocarbons (preferably chlorobenzene,dichlorobenzene and trichlorobenzene), aromatic hydrocarbons (preferablytoluene, xylene and benzene), ethers (preferably diphenyl ether),sulfoxides (preferably dimethylsulfoxide) and sulfones (preferablysulfolane). Particular preference is given here to chlorinated aromatichydrocarbons. Very particular preference is given to ortho-, meta- orpara-dichlorobenzene and also isomeric mixtures of dichlorobenzene.Exceptionally particular preference is given to ortho-dichlorobenzene.

The method according to the invention, surprisingly, is characterized bya low tendency to form secondary products which may lead to yield lossesand/or quality issues and/or phosgene development in virtuallyphosgene-free phases of the method.

In the method according to the invention, the proportion ofchlorine-containing species is at least significantly reduced in theproduct stream obtained after the removal as far as possible of lowboilers, hydrogen chloride and phosgene. This also applies to theby-products having chloroformamidine-N-carbonyl chloride structuralelements already mentioned, which can get into the virtuallyphosgene-free phases of the method as phosgene carrier and eliminatephogene again under thermal stress. Since the formation of by-productshaving chloroformamidine-N-carbonyl chloride structural elements isreduced as far as possible in the method according to the invention,this avoids that phosgene gets into virtually phosgene-free phases ofthe method.

Embodiments of the present invention are described in detail below,where the individual embodiments may be freely combined with oneanother, unless it is clearly implied to the contrary from the context.In addition, the liquid phase and gas phase processes are describedbelow. Which of the two method variants is to be preferred dependsparticularly on the amine to be used. Amines having a very high boilingpoint, wherein decomposition reactions are to be expected onevaporation, are preferably reacted by the liquid phase method. Amineswhich can evaporate without decomposition can basically be reacted byboth methods. The choice between liquid and gas phase methods in theselatter cases is dependent on various factors, not just technical, suchas economic constraints. The gas phase method is particularly preferredfor the amines selected from the group consisting of 1,6-diaminohexane,methylamine, ethylamine, propylamine, butylamine, pentylamine,hexylamine, cyclohexylamine, isophorone diamine,diaminodicyclohexylmethane (all isomers), mixtures ofdiaminodicyclohexylmethane isomers, benzylamine, aniline, chloroaniline,toluylenediamine (all isomers), 1,5-diaminonaphthalene anddiaminodiphenylmethane (all isomers).

The following description is based primarily on the aminetoluylenediamine. For those skilled in the art, it is easy to adjust themethod details given below, if necessary, to other amines.

The reaction of toluylenediamine with phosgene in step (i) may either becarried out by the liquid phase or the gas phase methods. The gas phasemethod is very particularly preferred in the case of toluylenediamine.

In the liquid phase method, the toluylenediamine is optionally alreadydissolved in one of the inert solvents defined above and is supplied tothe reaction chamber at temperatures of −10° C. to 220° C., preferably0° C. to 200° C., particularly preferably 20° C. to 180° C. The phosgenemay be fed to the reaction chamber, either without solvent or alsodissolved in one of the inert solvents defined above, at temperatures of−40° C. to 200° C., preferably −30° C. to 170° C., particularlypreferably −20° C. to 150° C. The TDA and phosgene, optionally alreadydissolved in one of the inert solvents defined above, are preferablymixed in the liquid phase method by means of a static mixer or a dynamicmixer. Examples of suitable static mixers are, inter alia, nozzles ornozzle arrangements described in, for example, DE 17 92 660 A, U.S. Pat.No. 4,289,732 or U.S. Pat. No. 4,419,295. Examples of suitable dynamicmixers are, inter alia, pump-like devices such as centrifugal pumps (cf.U.S. Pat. No. 3,713,833) or special mixer reactors (cf. EP 0 291 819 A,EP 0 291 820 A, EP 0 830 894 A).

In the liquid phase method, the reaction is carried out in the reactionchamber at temperatures of 0° C. to 250° C., preferably 20° C. to 200°C., particularly preferably 20° C. to 180° C., with mean residence timesof the reaction mixture in the reaction chamber of between 10 s and 5 h,preferably between 30 s and 4 h, particularly preferably between 60 sand 3 h, and at an absolute pressure of at most 100 bar, preferably 1.0bar to 70 bar, particularly preferably 1.0 bar to 50 bar. Examples ofmethod procedures which can be used according to the invention withrespect to the reaction in the reaction chamber are described, forexample, in US-A 2007/0299279 (particularly p. 7 paragraphs [0070],[0071], [0089]) and DE-A 103 10 888 (particularly p. 5 paragraphs[0038], [0039]) and documents cited therein.

In the gas phase method, the toluylenediamine is initially convertedinto the gas phase. This is preferably effected by means of anevaporator as is known from the prior art. The TDA is heated to 200° C.to 600° C., preferably 200° C. to 500° C., particularly preferably 250°C. to 450° C., optionally with an inert gas such as N2, He, Ar ordiluted with the vapor of one of the inert solvents defined above andfed to the reaction chamber. In the gas phase method, the phosgene,optionally diluted with an inert gas such as N2, He, Ar or with thevapor of one of the inert solvents defined above, is fed to the reactionchamber in gaseous form at temperatures of 200° C. to 600° C.,preferably 200° C. to 500° C., particularly preferably 250° C. to 450°C. The TDA and phosgene are preferably mixed in the gas phase by meansof static or dynamic mixing devices known to those skilled in the art.The use of nozzles is preferred, such as is described in EP 1 449 826B1, particularly in p. 4 paragraphs [0024], [0025], [0026] and p. 5paragraph [0027], or in EP 2 199 277 B1 in p. 4 paragraphs, [0017],[0018] and p. 5 paragraph [0019].

The reaction in the reaction chamber is effected in the gas phase methodat temperatures of 200° C. to 700° C., preferably 200° C. to 650° C.,particularly preferably 250° C. to 600° C. and with mean residence timesof the reaction mixture in the reaction chamber between 0.01 s and 120s, preferably between 0.01 s and 30 s, particularly preferably between0.05 s and 15 s, and at an absolute pressure of at most 5 bar,preferably 0.5 bar to 3.0 bar, particularly preferably 1.0 bar to 2.0bar. After leaving the reaction chamber, the hot gaseous reactionmixture is cooled by injecting, or passing through, one of the inertsolvents defined above at a temperature of 100° C. to 200° C.,preferably 150° C. to 180° C., and the isocyanate is liquefied. Examplesof method procedures which can be used in accordance with the inventionare described, inter alia, in EP 1 449 826 A1 (particularly p. 3paragraphs [0012], [0017], [0018], p. 4 paragraph [0022] and EP 2 199277 A1 (particularly p. 8 paragraphs [0054], [0055], [0056], [0057]).

In step (ii), the crude product 1 obtained in (i), comprising thedesired isocyanate, inert solvent, low boilers, high boilers, hydrogenchloride and unreacted phosgene, is separated into a liquid productstream 2 and a gaseous product stream 3, which largely compriseshydrogen chloride and excess phosgene. This is preferably carried out inapparatus and containers known to those skilled in the art which aresuitable for separating gas and liquid phases. Preference is given tousing gas and liquid separators such as cyclone separators, baffleseparators, gravitational separators with or without static separationaids.

The liquid product stream 2 obtained in step (ii) comprises, in additionto the isocyanate and the inert solvent used, low-boiling substancessuch as phosgene and hydrogen chloride, and also high boilers and lowboilers. The liquid product stream 2 generally comprises between 10% byweight and 100% by weight of toluylene diisocyanate, between 0% byweight and 90% by weight of inert solvent, between 0% by weight and 5.0%by weight of high boilers, between 10% by weight and 5.0% by weight oflow boilers, between 0% by weight and 5.0% by weight of phosgene andbetween 0% by weight and 5.0% by weight of dissolved hydrogen chloride,based in each case on the total weight of the liquid product stream 2.

Depending on the configuration of step (ii), a plurality of substreams2a, 2b, etc. may also be obtained, preferably two substreams, 2a and 2b,are obtained. This is the case when the crude product 1 is initiallypassed into a device for separating gas and liquid phases, wherein aliquid phase 2a and a gas phase is formed, and the gas phase issubsequently passed through a device for droplet separation, where aliquid phase 2b and a gas phase 3 freed of entrained droplets is formed.The additional stream 2b preferably comprises 0% by weight to 5.0% byweight of isocyanate, 0% by weight to 10% by weight of phosgene and 0%by weight to 100% by weight of solvent. The composition of the mainstream 2a preferably corresponds to the composition of 2 mentionedabove.

If just one liquid product stream 2 is obtained in step (ii), this isprocessed to give pure isocyanate in step (iii). If two or more liquidproduct streams 2a, 2b, etc. are obtained in step (ii), these are eithercombined and processed together to pure isocyanate, or these arepartially combined and processed further in two or more substreams, oreach substream is further processed separately, wherein at least one ofthe substreams is processed to pure isocyanate. It is also possible hereto combine the substreams at a later time point in the course of theirfurther processing. The requirements of the present invention for theperiod up to the separation of the low boilers, hydrogen chloride andphosgene and the temperature in step (iii) apply equally to each of theproduct streams 2a, 2b, etc.

The description of step (iii) below is based on the embodiment withexactly one liquid product stream 2. The method details applyanalogously, however, to the processing of multiple substreams 2a, 2b,etc., with optional adjustments self-evident to those skilled in theart.

The temperature of the liquid product stream 2 obtained in step (ii) isgenerally at most 250° C., preferably at most 220° C. and particularlypreferably at most 200° C. Furthermore, the temperature of 2 istypically at least 60° C., preferably at least 90° C. and particularlypreferably at least 110° C. These values apply equally to the liquid andthe gas phase methods.

Furthermore, a gaseous product stream 3 largely comprising phosgene andhydrogen chloride is obtained in step (ii). The composition of thisstream depends on the exact process parameters in step (i) and (ii).Detailed knowledge of the composition of 3 is not relevant forunderstanding the invention. In the context of the present invention,the product stream 3 is further processed as is customary from the priorart. Among other things, phosgene is separated and recycled in theprocess. Hydrogen chloride is also separated and purified as required inorder to be able to use it, for example, as aqueous hydrochloric acidafter absorption, or to close the chlorine circuit, in which hydrogenchloride, for example, is reconverted to chlorine by means ofelectrolysis or by oxidation in a Deacon process.

The workup of the liquid product stream 2 in step (iii) is preferablycarried out by distillation. Alternatively, other methods, such ascrystallization, extraction or membrane methods may also be used for theworkup. Of course, a combination of various methods for the workup isalso possible.

The preferred workup by distillation of the liquid product stream 2 instep (iii) may be carried out either in a single stage or particularlypreferably in multiple stages. Suitable apparatuses are, for example,columns which are optionally provided with suitable internals and/orrandom packings, separating trays and/or structured packings. Preferenceis given to dividing wall columns. Suitable combinations of multiplecolumns or column types are also possible. Using a dividing wall column,two or more separation processes and tasks for the workup may becombined in one apparatus. In general, the workup by distillation isconducted at bottom temperatures in the range of 60° C. to 250° C.,preferably 100° C. to 240° C., and particularly preferably 120° C. to230° C. The absolute head pressures are generally in the range of 1.0mbar to 1400 mbar, preferably 5.0 mbar to 1013 mbar. The absolute bottompressures are higher than the head pressures and are in the range of 2.0mbar to 2000 mbar, preferably 7.0 mbar to 1500 mbar.

The workup in step (iii), preferably by distillation, comprisesobtaining the product of value toluylene diisocyanate (TDI) by removingthe inert solvent, the low boilers, the high boilers, the phosgene andthe hydrogen chloride. The inert solvent is preferably purified asrequired and recycled into the process. Hydrogen chloride may also bepresent chemically bound, e.g. as carbamoyl chloride and also in theform of other chlorine-containing species. The species with chemicallybound hydrogen chloride are generally in the group of the high boilers.Carbamoyl chloride in step (iii) is cleaved as far as possible to obtainthe isocyanate.

It is essential to the invention that the low boilers, the hydrogenchloride and the phosgene in step (iii) are removed from the desiredisocyanate within a period of 30 seconds to 60 minutes, preferably 60seconds to 50 minutes, particularly preferably 2 minutes to 40 minutes,following the separation of the crude product 1 in step (ii) into theproduct streams 2 and 3, in the present exemplary description for TDI,and the temperature of the product stream 2 is always maintained belowor equal to 250° C., preferably between 100° C. and 250° C.,particularly preferably between 120° C. and 230° C. The lower limit ofsaid period is on the one hand governed by the spatial distance betweenreaction and workup, preferably distillation, and on the other hand, ofcourse, by the fact that a minimum residence time in the selectedseparating device, preferably a distillation column, is necessary toremove the low-boiling substances mentioned.

The removal of the low boilers, the hydrogen chloride and the phosgeneshall be carried out for the purposes of the invention if in each caseat least 95.0% by weight, preferably at least 98.0% by weight,particularly preferably at least 99.0% by weight, especially preferablyat least 99.5% by weight and exceptionally preferably 100% by weight ofthe low boilers, the hydrogen chloride and the phosgene are removed,based in each case on the weight of the low boilers, the weight ofhydrogen chloride and the weight of the phosgene, which leaves thereaction chamber.

In a preferred embodiment of the workup in step (iii), said workup iscarried out in more than one stage, wherein inert solvent, low boilers,hydrogen chloride and phosgene are removed from the product stream 2 bydistillation in a first stage (iii.a), such that a product stream 4depleted in inert solvent, low boilers, hydrogen chloride and phosgeneis obtained, and in at least one further stage (iii.b) pure isocyanate 5is obtained from the product stream 4 by distillation. These steps(iii.a) and (iii.b) may be carried out in apparatuses known from theprior art. It is only essential here that the low boilers, the hydrogenchloride and the phosgene in step (iii.a) are removed within a period of30 seconds to 60 minutes, preferably 60 seconds to 50 minutes,particularly preferably 2 minutes to 40 minutes, following theseparation of the crude product 1 in step (ii) into the product streams2 and 3 and the temperature of the product stream 2 is always maintainedbelow or equal to 250° C., preferably between 100° C. and 250° C.,particularly preferably between 120° C. and 230° C. The dimensions andarrangement of the individual apparatuses, the sizes of connectingpiping and flow rates must therefore be coordinated, such that theperiod up to the removal of the low boilers, the hydrogen chloride andthe phosgene is maintained in accordance with the invention. A personskilled in the art is able to determine suitable parameters in a simplemanner, if required, simple preliminary experiments may be necessary fora given production system. A so-called dephosgenation column is used forstep (iii.a). A distillation column or dividing wall column known tothose skilled in the art is preferably used for step (iii.b) for finalpurification. To obtain maximum purity isocyanate, multiple distillationcolumns may also be used in which different column types may becombined. Suitable apparatus for carrying out the steps (iii.a) and(iii.b) are described, for example, in Ullmann's Encyclopedia ofIndustrial Chemistry (Johann Stichlmair; Distillation, 2. Equipment;published online 2010-04-15, DOI: 10.1002/14356007.o08_o01), EP 1 546091 A1, U.S. Pat. No. 2,471,134 A, US 2003/0047438 A1, EP 1 371 633 A1,EP 1 371 634 A1, EP 1 413 571 A1, EP 2 210 873 A1, WO 2010/039972 A2, DE19 23 214 A1, EP 1 475 367 B1. How such apparatus are to be basicallyoperated is known to those skilled in the art.

In the preferred multi-stage workup in steps (iii.a) and (iii.b), thetime span to be observed according to the invention between theseparation of the crude product 1 into the liquid product stream 2 andthe gaseous product stream 3 corresponds to the mean residence time ofisocyanate formed in step (i) between the effluent of stream 2 from adevice for separating liquid and gas phases (step (ii)) and the effluentof product stream 4 depleted in inert solvent, low boilers, hydrogenchloride and phosgene from the dephosgenation column (step (iii.a)). Thedevice for separating liquid and gas phases may be integrated in thereactor from step (i), such that the time span to be observed accordingto the invention between the separation of the crude product 1 into theliquid product stream 2 and the gaseous product stream 3 corresponds tothe mean residence time of isocyanate formed in step (i) between theeffluent of stream 2 from the reactor and the effluent of product stream4 depleted in inert solvent, low boilers, hydrogen chloride and phosgenefrom the dephosgenation column. In this variant, the invention thereforerelates to a method in which step (iii.a) is carried out in adephosgenation column and step (iii.b) is carried out in a column forfinal purification, and in which the residence time of isocyanate formedin step (i) after the separation of the crude product 1 in step (ii) upto the effluent of the product stream 4 depleted in inert solvent, lowboilers, hydrogen chloride and phosgene from the dephosgenation columnin step (iii.a) is 30 seconds to 60 minutes, preferably 60 seconds to 50minutes, particularly preferably 2 minutes to 40 minutes.

In this manner, a liquid product stream 4 depleted in inert solvent, lowboilers, hydrogen chloride and phosgene is obtained. Stream 4 preferablycomprises 40% by weight to 100% by weight, particularly preferably 60%by weight to 100% by weight, especially preferably 70% by weight to 100%by weight of the desired isocyanate, in this exemplary description forTDI, based in each case on the total weight of this stream,

0.10% by weight to 10% by weight, preferably 0.50% by weight to 7.0% byweight, particularly preferably 1.0% by weight to 5.0% by weight of highboilers,

0% by weight to 60% by weight, particularly preferably 0% by weight to40% by weight, especially preferably 0% by weight to 30% by weight ofsolvent,

0 ppm by weight to 1500 ppm by weight (0-2%), preferably 0 ppm by weightto 1200 ppm by weight (0-1.2%), particularly preferably 0 ppm by weightto 1000 ppm by weight (0-1%) of low boilers,

0 ppm by weight to 1000 ppm by weight, preferably 0 ppm by weight to 800ppm by weight, particularly preferably 0 ppm by weight to 600 ppm byweight of phosgene,

0 ppm by weight to 1000 ppm by weight, preferably 0 ppm by weight to 800ppm by weight, particularly preferably 0 ppm by weight to 600 ppm byweight of hydrogen chloride.

This stream 4 is subsequently further processed by distillation in orderto obtain pure isocyanate 5, in the present exemplary description forpure TDI. This purifying distillation may be conducted by all typesknown from the prior art. Examples are described, for example, in EP 1371 634 A1 or the literature references or applications cited therein.

In the embodiment described above by way of example, toluylenediisocyanate (TDI) is obtained as product, in which the isomericdistribution substantially corresponds to that of the toluylene diamineused.

If other amines are reacted to the corresponding isocyanates,modifications are optionally carried out to the individual methoddetails described above, which is a routine operation, however, to thoseskilled in the art.

By the inventive limitation of the temperature and the time periodbetween the separation of the crude product 1 into the product streams 2and 3 in stop (ii) and the removal of the low boilers, the hydrogenchloride and the phosgene from the desired isocyanate in step (iii), thethermally induced formation of carbodiimides on the one hand and theformation of secondary components having chloroformamidine-N-carbonylchloride structural elements on the other hand is largely avoided. Suchsecondary components could otherwise lead to yield losses and/or qualityissues and/or phosgene development in virtually phosgene-free methodphases. The method according to the invention thus reduces yield lossesand therefore economic disadvantages to a minimum.

EXAMPLES

The following examples demonstrate the importance of the removal of lowboilers, hydrogen chloride and phosgene within the time period accordingto the invention. To demonstrate the importance of these parameters, thecontent of chemically bound phosgene was determined in all isocyanatesamples. “Chemically bound” is, for example, phosgene present incarbodiimides. The greater the content of chemically bound phosgene in acrude isocyanate stream, the less selective is the reaction of the amineand the greater is the risk that phosgene is “kidnapped” intophosgene-free phases of the method.

In all examples, a mixture of 80% 2,4-TDA and 20% 2,6-TDA wasphosgenated in each case in a gas phase method as described in EP 0 570799 B1 (step (i)).

The crude product 1 was separated in a reaction quench zone into aliquid product stream 2 and a gaseous product stream 3 by injection ofortho-dichlorobenzene (ODB) as inert solvent (step (ii)).

The liquid reactor effluent 2 thus obtained comprised 87 ppm by weight(0.0087% by weight) chemically bound phosgene. To determine the contentof chemically bound phosgene the following procedure was followed in allexamples:

All samples were initially freed from the presence of potential residualamounts of dissolved phosgene by purging with dry nitrogen (40 l/h) for8 hours at a maximum of 30° C. 200 g of the samples thus prepared werethen heated for 30 min at 180° C. with stirring in a round-bottomedflask equipped with gas inlet line and reflux condenser and maintainedfor a further 90 min at this temperature with stirring. A nitrogen flowof 10 l/h was passed through the crude product solution throughout theentire experimental run in order to expel chemically bound phosgenesubsequently liberated by thermal stress from the solution and to conveyit to a cascade of wash bottles comprising methanol in defined amounts.Phosgene reacts with the methanol to give dimethyl carbonate, which canbe analyzed quantitatively by gas chromatography (GC). For precisequantification, benzophenone was added to the methanol as internalstandard for the GC analysis.

Example 1 Inventive

A sample of the liquid reactor effluent 2 was transferred continuouslyto a dephosgenation column and freed from inert solvent, low boilers,hydrogen chloride and phosgene (step (iii.a)), wherein a pre-purifiedisocyanate stream 4 was obtained as bottom product from thedephosgenation column

Temperature of the stream 2 on entering the dephosgenation column

163° C.

Temperature at the bottom of the dephosgenation column: 180° C.

Absolute pressure at the head of the dephosgenation column: 680 mbar

The mean residence time between effluent of stream 2 from the reactorand effluent of stream 4 from the bottom of the dephosgenation columnwas ca. 20 min. Based on TDI, stream 4 contained 58 ppm by weight(0.0058% by weight) of chemically bound phosgene. This example showsthat at a residence time in accordance with the invention betweenobtaining the liquid product stream 2 and the removal of the lowboilers, the phosgene and the hydrogen chloride, the content ofchemically bound phosgene is practically unchanged (the nominal valuewas even lower, due to measurement variations).

Example 2 Comparative Example

A further sample of the liquid reactor effluent 2 was treated undercontinuous feed of phosgene for 3 h at 180° C. and standard pressure, torecreate the conditions of a non-inventive residence time betweenobtaining the stream 2 and removal of the low boilers, the phosgene andthe hydrogen chloride as realistically as possible.

Based on TDI, the sample contained 485 ppm by weight (0.0485% by weight)of chemically bound phosgene.

The examples demonstrate that the content of chemically bound phosgenein the inventive procedure is not adversely affected while at a largerresidence time between obtaining the stream 2 and removal of the lowboilers, the phosgene and the hydrogen chloride, the content ofchemically bound phosgene significantly increases.

What is claimed is:
 1. A continuous method for preparing an isocyanate by (i) reacting the corresponding primary amine with phosgene in stoichiometric excess in a reaction chamber, wherein the reaction is carded out either in the liquid phase in the presence of an inert solvent or in the gas phase, wherein a stream comprising a liquid inert solvent is added to the process product after leaving the reaction chamber, such that a crude product 1 is obtained comprising the desired isocyanate, inert solvent, secondary components having a boiling point below that of the isocyanate (low boilers), secondary components having a boiling point above that of the isocyanate (high boilers), hydrogen chloride and unreacted phosgene, (ii) separating the crude product 1 into a liquid product stream 2 containing the desired isocyanate and into a gaseous product stream 3, (iii) working-up the liquid product stream 2, wherein inert solvent, low boilers, high boilers, hydrogen chloride and phosgene are removed from the desired isocyanate, characterized in that the low boilers, the hydrogen chloride and the phosgene are removed in step (iii) within a period of 30 second to 60 minutes following the separation of the crude product 1 in step (ii) into the product streams 2 and 3, and the temperature of the product stream 2 is always maintained below or equal to 250° C.
 2. The method as claimed in claim 1, in which the workup step (iii) is carried out in more than one stage, wherein inert solvent, low boilers, hydrogen chloride and phosgene are removed from the product stream 2 by distillation in a first stage (iii.a), such that a product stream 4 depleted in inert solvent, low boilers, hydrogen chloride and phosgene is obtained, and in at least one further stage (iii.b) pure isocyanate 5 is obtained from the product stream 4 by distillation.
 3. The method as claimed in claim 2, in which step (iii.a) is carried out in a dephosgenation column and step (iii.b) is carried out in a column for final purification, and in which the residence time of isocyanate formed in step (i) after the separation of the crude product 1 in step (ii) up to the effluent of the product stream 4 depleted in inert solvent, low boilers, hydrogen chloride and phosgene from the dephosgenation column in step (iii.a) is 30 seconds to 60 minutes.
 4. The method as claimed in any of claims 1 to 3, in which the reaction of the primary amine with phosgene in the reaction chamber in step (i) is carried out in the gas phase.
 5. The method of claim 1, in which the primary amine used is toluylenediamine.
 6. A continuous method for preparing an isocyanate by (i) reacting the corresponding primary amine with phosgene in stoichiometric excess in a reaction chamber, wherein the reaction is carried out in the gas phase, wherein a stream comprising a liquid inert solvent is added to the process product after leaving the reaction chamber, such that a crude product 1 is obtained comprising the desired isocyanate, inert solvent, secondary components having a boiling point below that of the isocyanate (low boilers), secondary components having a boiling point above that of the isocyanate (high boilers), hydrogen chloride and unreacted phosgene, (ii) separating the crude product 1 into a liquid product stream 2 containing the desired isocyanate and into a gaseous product stream 3, (iii) working-up the liquid product stream 2, wherein inert solvent, low boilers, high boilers, hydrogen chloride and phosgene are removed from the desired isocyanate, characterized in that the low boilers, the hydrogen chloride and the phosgene are removed in step (iii) within a period of 30 second to 60 minutes following the separation of the crude product 1 in step (ii) into the product streams 2 and 3, and the temperature of the product stream 2 is always maintained below or equal to 250° C.
 7. The method of claim 6, in which the workup step (iii) is carried out in more than one stage, wherein inert solvent, low boilers, hydrogen chloride and phosgene are removed from the product stream 2 by distillation in a first stage (iii.a), such that a product stream 4 depleted in inert solvent, low boilers, hydrogen chloride and phosgene is obtained, and in at least one further stage (iii.b) pure isocyanate 5 is obtained from the product stream 4 by distillation.
 8. The method of claim 7, in which step (iii.a) is carried out in a dephosgenation column and step (iii.b) is carried out in a column for final purification, and in which the residence time of isocyanate formed in step (i) after the separation of the crude product 1 in step (ii) up to the effluent of the product stream 4 depleted in inert solvent, low boilers, hydrogen chloride and phosgene from the dephosgenation column in step (iii.a) is 30 seconds to 60 minutes.
 9. The method of claim 6, in which the primary amine used is toluylenediamine. 